Betasitosterolemic patients have xanthomas in childhood, markedly premature artherosclerotic cardiovascular disease, and elevated levels of plant sterols, primarly betasitosterol. We have shown that in addition to over absorbing dietary plant sterols, they also over absorb shellfish sterols. This suggests that betasitosterolemic patients have a generalized overabsorption of dietary sterols. The molecular defect in this disease appears to be due to an abnormality at the site for discrimination for sterol absorption. In normal individuals plant and shellfish sterols are not absorbed whereas cholesterol is actively transported into the body. In patients with betasitosterolemia there is no selectivity to sterol absorption, and cholesterol, plant sterols and shellfish sterols are absorbed in significant quantities.